We are studying the properties of a human protein, designated EBP-80, which we isolated by virtue of its sequence-specific binding to the intracisternal A-particle long terminal repeat element (IAP LTR). The preparation proved to be very similar or identical to Ku, a much- studied protein originally identified as an autoantigen in human patients. EBP-80/Ku is a heterodimeric protein consisting of 70 and 86 kDa subunits and binds tightly to ends of duplex DNA. In earlier reports, we described work showing that end-binding is a special case of the more general capacity of EBP-80/Ku to recognize and bind to junctions between single- and double-stranded DNA. Ku is known to be the DNA-binding and activating subunit of a DNA-dependent serine/ threonine protein kinase (DNA-PK), and we showed that this enzyme is also activated by constructs containing single-to-double strand transitions. Ku and the catalytic subunit of DNA-PK are both required for repair of double strand DNA breaks and V(D)J recombination. Dr. Morozov has made the novel observation that Ku has a 5' to 3' single- strand exonuclease activity which digests single-strand extensions from duplex regions. Such an activity has been postulated in models of the repair process but not previously known to be associated with any of the recognized components. Immunocytochemical studies of cultured human and monkey cells (J. Fewell), using monoclonal and polyclonal antibodies against the individual Ku subunits, have revealed striking changes in epitope accessibility as the cultures progress from sparse to confluent cell densities. The same changes are elicited within 15 min. by exposure of cells to serum- or Ca2+-free media, or to picomolar amounts of TGF- beta. The results suggest that Ku participates in a signalling system sensitive to cell-cell contact and to other specific external growth- inhibitory stimuli.